In recent years, a high-speed and large-capacity data transmission has been demanded in a wireless communication system. Thus, research for increasing the use efficiency of a limited frequency band has been actively conducted. As a technique for achieving the purpose, high-speed communications using a multi-carrier transmission scheme such as an OFDM (Orthogonal Frequency Division Multiplex) scheme can be cited. However, use of such a multi-carrier transmission scheme has a problem of making PAPR (Peak to Average Power Ratio) high although achieving very high use of frequency.
This problem with the PAPR does not cause a large trouble in downlink where there is a margin in a transmission power amplification function, but causes an extremely large trouble in uplink where amplifiers have limitations. For this reason, use of a single carrier transmission scheme having a low PAPR is preferable for the uplink.
In this respect, for an LTE (Long Term Evolution) system which is the wireless communication system for the 3.9-generation mobile phones, a DFT-S-OFDM (Discrete Fourier Transform-Spread-OFDM (also, referred to as SC-FDMA)) scheme is employed as one of transmission schemes having a low PAPR in the uplink and being capable of generating a waveform by use of the same signal generation technique as that of the OFDM scheme. Like the OFDM scheme, the DFT-S-OFDM scheme enables frequency control in the following manner, although the DFT-OFDM scheme is a single carrier scheme. In the DFT-S-OFDM scheme, a modulation symbol sequence formed in blocks is transformed into a frequency signal by FFT (Fast Fourier Transform), and discrete spectrums are arranged on the basis of a specific arrangement rule to regenerate a time signal by IFFT (Inverse FFT: Inverse Fast Fourier Transform).
Further, for LTE-A (LTE-Advanced (also referred to as “IMT-A”)) which is the fourth generation wireless communication system currently in progress for standardization, Clustered DFT-S-OFDM (also referred to as Clustered SC-FDMA or DSC (Dynamic Spectrum Control)) has been proposed as a technique to increase the frequency use efficiency while suppressing an increase in the PAPR (for example, Non-Patent Documents 1 and 2). Here, a cluster is a term for a group of multiple consecutive subcarriers. Specifically, the Clustered DFT-S-OFDM includes: dividing frequency components of a DFT-S-OFDM signal into some clusters; and then rearranging the frequency components. Use of this scheme enables the frequency band to be used flexibly in accordance with a propagation channel characteristic, although this scheme slightly deteriorates the PAPR property as compared with DFT-S-OFDM.
FIG. 5 is a diagram showing a configuration example of a transmitter using the Clustered DFT-S-OFDM scheme. The transmitter shown in FIG. 6 includes the following units cited in the order from the input side of an inputted transmission bit sequence: an encoder 1000; an interleave unit 1001, a modulator 1002, a serial-to-parallel (S/P) converter 1003, a Discrete Fourier Transform (DFT) unit 1004, a cluster mapping unit 1005, an Inversed DFT (IDFT) unit 1006, a parallel-to-serial (P/S) converter 1007, a pilot signal generator 1008, a pilot multiplexer 1009, a cyclic prefix (CP) addition unit 1010, a digital-to-analog (D/A) converter 1011, a radio unit 1012 and a transmission antenna 1013.
Firstly, the encoder 1000 encodes a transmission bit sequence. The encoded data pieces are rearranged by the interleave unit 1001, and then modulated by the modulator 1002. The modulation signal is converted from serial data into parallel data by the S/P converter 1003, and then inputted to the DFT unit 1004. The inputted data is transformed into a signal on the frequency axis by the discrete Fourier transform. Thereafter, the cluster mapping unit 1005 divides the signal by determined cluster size, and maps each clustered signal to a subcarrier having an optimum frequency for the signal. Here, the transmitter is given, by a receiver, feedback information for specifying a mapping assignment in accordance with a receiving condition or the like, and performs the mapping on the basis of the information. The mapped symbol sequence is again transformed into a time axis signal by the IDFT unit 1006, and is further reconverted into serial data by the P/S converter 1007. Here, for the purpose of estimating a frequency characteristic of a propagation channel, a pilot signal is generated in parallel by the pilot signal generator 1008, and then the pilot signal and the aforementioned serial data are multiplexed by the pilot multiplexer 1009. A cyclic prefix (which may be considered as equivalent to a guard interval (GI)) is added to the multiplexed signal by the CP addition unit 1010. Then, after being converted into an analog signal in the D/A converter 1011, the signal is up-converted into a radio frequency to be used in the radio unit 1012. Then, the transmission antenna 1013 transmits the signal.
Meanwhile, as a technique to improve an error rate of a received signal, a cyclic delay diversity (CDD) technique has been proposed on the assumption that the receiver can perform frequency domain equalization. In this technique, the top time positions of impulse responses are equivalently shifted by cyclically delaying signals from transmission antennas, and thereby the number of multipaths is intentionally increased to enhance the frequency selectivity of a propagation channel. In this way, this technique obtains a larger frequency diversity effect. FIG. 6 is a diagram showing the concept of the CDD. As shown in FIG. 6, the same signal is transmitted with different cyclic delays from multiple antennas. Accordingly, the number of incoming waves of the same signal increases in the receiver. As a result, the number of multipaths in the propagation channel is equivalently increased. Thus, the frequency diversity effect can be expected.